When I run qemu-kvm with the options to forward a port from the host to a port on the guest, such as:

qemu-kvm -net nic,model=virtio -net user,hostfwd=tcp::7272-:22 arch.img

And, in parallel, as soon qemu-kvm is running, while the guest is still in the early bootloader stages, I run (still on the host):

telnet localhost 7272

Then qemu-kvm segfaults a couple of seconds later when the guest is apparently trying to set networking up; see the GDB session attached for a backtrace.

In fact, this happens deterministically when anything (telnet, ncat, ssh, etc.) tries to connect to the guest early on, and it does not matter either whether something is actually listening on port 22 of the guest.